From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44920) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eopbM-0007CV-C6 for qemu-devel@nongnu.org; Thu, 22 Feb 2018 07:03:58 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eopbG-0000a7-6u for qemu-devel@nongnu.org; Thu, 22 Feb 2018 07:03:52 -0500 Date: Thu, 22 Feb 2018 12:03:34 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20180222120334.GJ9323@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <7b01ba4b-7f5f-30c7-d6d8-3c22f2774e7e@kamp.de> <7c23f28d-ee14-c504-ae13-04414c4b032e@redhat.com> <20180222105755.GB4147@localhost.localdomain> <37efa6ac-8e36-9c96-84ef-a8639ba64937@kamp.de> <20180222113204.GC4147@localhost.localdomain> <20180222114053.GE9323@redhat.com> <20180222120029.GI9323@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [Qemu-block] Limiting coroutine stack usage List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven Cc: Kevin Wolf , Paolo Bonzini , jsnow@redhat.com, "qemu-devel@nongnu.org" , qemu block On Thu, Feb 22, 2018 at 01:02:05PM +0100, Peter Lieven wrote: > Am 22.02.2018 um 13:00 schrieb Daniel P. Berrang=C3=A9: > > On Thu, Feb 22, 2018 at 12:51:58PM +0100, Peter Lieven wrote: > >> Am 22.02.2018 um 12:40 schrieb Daniel P. Berrang=C3=A9: > >>> On Thu, Feb 22, 2018 at 12:32:04PM +0100, Kevin Wolf wrote: > >>>> Am 22.02.2018 um 12:01 hat Peter Lieven geschrieben: > >>>>> Am 22.02.2018 um 11:57 schrieb Kevin Wolf: > >>>>>> Am 20.02.2018 um 22:54 hat Paolo Bonzini geschrieben: > >>>>>>> On 20/02/2018 18:04, Peter Lieven wrote: > >>>>>>>> Hi, > >>>>>>>> > >>>>>>>> I remember we discussed a long time ago to limit the stack usa= ge of all > >>>>>>>> functions that are executed in a coroutine > >>>>>>>> context to a very low value to be able to safely limit the cor= outine > >>>>>>>> stack size as well. > >>>>>>> IIRC the only issue was that hw/ide/atapi.c has mutual recursio= n between > >>>>>>> ide_atapi_cmd_reply_end -> ide_transfer_start -> ahci_start_tra= nsfer -> > >>>>>>> ide_atapi_cmd_reply_end. > >>>>>>> > >>>>>>> But perhaps it's not an issue, somebody needs to audit the code= . > >>>>>> I think John intended to get rid of the recursion sometime, but = I doubt > >>>>>> he has had the time so far. > >>>>> Apart from this is is possible to define special cflags in the > >>>>> Makefile.objs just for a subdirectory? I have patches ready to ma= ke > >>>>> the block layer files and other coroutine users compile with > >>>>> -Wstack-size=3D2048. But I do not want to specify each file separ= ately. > >>>> Our Makefiles have lines like this: > >>>> > >>>> iscsi.o-cflags :=3D $(LIBISCSI_CFLAGS) > >>>> > >>>> I don't think there is a direct mechanism to apply cflags to a who= le > >>>> directory or just to block-obj-y/block-obj-m, but just looping ove= r them > >>>> could work. I'm not a Makefile expert at all, but after some toyin= g with > >>>> a simple example, something like this might work: > >>>> > >>>> $(foreach x,$(block-obj-y),$(eval $x-cflags +=3D -Wstack-size=3D= 2048)) > >>> You'll need it for anything block layer depends on too - so that's = much > >>> of util/, crypto/ and io/ directories at least. > >>> > >>> So perhaps it would be shorter if we do the opposite - set -Wstack-= size=3D2048 > >>> globally for everything in QEMU, and then override -Wstack-size=3D$= BIGGER > >>> for the (hopefully) few sources that have a larger stack need ? > >> I tried that already. 2048 is a strong limit for many functions. > >> It breaks already as soon as some buffer has a size of PATH_MAX, but > >> thats handleable. But there are some structs around that are very la= rge. > > There are surprisingly few "char [PATH_MAX]" variables left in QEMU -= we > > should have a final push to eliminate them regardless. > > > >> Generally, it would be a good idea to have a global limit, of course= . > > We could at least put a limit on that matches the current worst case = to > > prevent it getting worse than it already is. >=20 > That would be a good idea, yes. >=20 > How would you handle the override for a smaller -Wstack-usage ? If you have multiple -Wstack-size=3D$XXX flags to GCC, I expect the last one wins. So just need to double check that the per-object file CFLAGS occur after the global CFLAS in the compiler args Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|